Well that's the thing with Wiccan magic, it isn't considered to be supernatural at all. It's really quite comparable to prayer for Christians, with the exception that Wiccans believe that The Divine put natural energy in this world for humans to effect their own lives,
The irony is that this can also be supported by the Bible, however, it's always under the direction or approval of The Most High. It seems that the difference is more so in the medium. When miracles are performed in the bible by those who are chosen by The Most High... they're called miracles. Otherwise, such acts are called witchcraft.
There seems to be evidence even in Christianity supporting such things, and the question isn't so much what is done but rather how and why... by what force and for what purpose.
rather than believing that God has a personal hand in each little thing that happens. IF a Wiccan was in a severely drought-plagued area and decided to try to bring rain, they would be trying to move natural energies, provided by The Divine (God and Goddess), to make that happen rather than praying to God for it to happen.
And plenty of Christians pray for rain.
From a Christian standpoint, I can see why someone might think that is akin to trying to be a God in your own right. Most Christians believe that God has a plan for everything, and though that doesn't stop them from praying for things, they do not give themselves any credit for what happens. That's God's doing. People even thank God when they win a football game, no matter how hard they themselves worked to achieve that goal.
Although I'm not Wiccan, I think the same can be said for both in a manner of speaking. Christians should, and are Scripturally encouraged to, recognize their own strength. However, they are also reminded that it is The Most High who gives strength.
Personal power becomes sketchy not so much because we play like we're mini-gods, but even more so because we do not have perfect judgment. And the means we use to make things manifest might be questionable. More specifically, there is no expressed writing saying that using personal power is forbidden. It is said that we can do anything in HIS name. At the same time, we're not to contact other deities or the dead, not for prayer or anything else. So basically, the difference would be in who we petition to and the intent of our actions, and not so much what is being done. There in lies the danger of exacting ones will, regardless if one uses a spell or not. And I honestly think that people forget to apply the same rules inside and outside of "magic". Meaning, people tend to be hypocritical when they make remarks about "magic."
If I say a prayer with or without a candle, it does not change the fact that I have to have faith in what I asked, a strong desire for what I ask, and the willingness to make it happen. Oddly, that is something that is often not mentioned about prayer in Christianity, even though it is in the Bible. HOW to pray is rarely stressed, and yet there is a religion full of people who "believe" in prayer yet don't necessarily feel that it works.
Honestly, I feel that they work in the same way, for the most part.
The other difference, I feel, has to do with Law & Grace. But because Christian understanding of that is so varied, I won't bother going into it.
In Wicca, we believe that we are masters of our own destinies, that natural energy can be used to effect change in the world and our own lives. Short of providing the energy for everyone to utilize, the Gods don't really take much of a personal hand in our daily existence (depending on your specific belief system). So, when I change my life or the world, in a small or large way, I can take credit for that change rather than thanking some unknowable deity. To a Christian, that seems a lot like taking credit for God's work, or "playing God." But to me, it only makes sense to take credit for my own successes and failures.
To me, this seems like something that is true on the surface, but is often untrue in people's hearts. A lot of people say "thank you" and seem to give credit as a taught behavior. I don't know too many people who say that The Most High did something good for them, who don't take personal credit. By the same token, I've also seen many Christians who have blamed G-D for their troubles as well. It just seems to vary a lot according to the individual, their understanding of religion/spirituality, and the strength of their faith... not so much to do with Scriptural teaching.